Adeno-associated virus (AAV) vector-based gene therapies can be applied to a wide range of diseases. AAV expression can last for months to years, but vector re-administration may be necessary to achieve life-long treatment. Unfortunately, immune system response against these vectors is potentiated after the first administration, which prevents the clinical use of repeated administration of AAVs. Reducing immune response against AAVs while minimizing immunosuppression would improve gene delivery efficiency and long-term safety. In this study, we quantified the contributions of multiple immune system components towards AAV response in mice. We identified B-cell-mediated immunity as a critical component preventing vector re-administration. Specifically, we found that IgG depletion was insufficient to enhance re-administration, suggesting the key role of B-cell mediated IgM antibodies in the immune response against AAV. Further, we also found that AAV-mediated transduction is improved compared to wild-type mice in µMT mice that lack functional IgM heavy chains and cannot form mature B-cells. Combined, our results suggest that IgM production in B cells is a potential target for therapeutics enabling AAV re-administration. Our results also suggest that the µMT mice are a potentially useful experimental model for gene delivery studies since they allow for up to 15-fold more efficient gene delivery.